<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
            margin: 0;
            padding: 0;
        }
        .box{
            width: 100vw;
            height: 100vh;
        }
        .big {
            width: 100px;
            height: 100px;
            background-color: aqua;
        }

        .flex {
            display: flex;
            justify-content: center;
            align-items: center
        }
        .position{
            position: absolute;
            top: 50%;
            left: 50%;
            transform:translate(-50%,-50%)
        }
        .position1{
            position: absolute;
            top: 0;
            left: 0;
            right: 0;
            bottom: 0;
            margin: auto;
        }
    </style>
</head>

<body>
    <div class="box">
        <div class="big position1">
        </div>
    </div>
</body>

</html>